Network Working Group J. Peterson
Internet-Draft Neustar
Intended status: Standards Track S. Turner
Expires: September 14, 2017 sn3rd
March 13, 2017
OCSP Usage for Secure Telephone Identity Certificates
draft-ietf-stir-certificates-ocsp-00.txt
Abstract
When certificates are used as credentials to attest the assignment or
ownership of telephone numbers, some mechanism is required to convey
certificate freshness to relying parties. This document specifies
the use of the Online Certificate Status Protocol (OCSP) as a means
of retrieving real-time status information about such certificates,
defining new extensions to compensate for the dynamism of telephone
number assignments.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on September 14, 2017.
Copyright Notice
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document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Certificate Verification Methods . . . . . . . . . . . . . . 3
3.1. Using OCSP with TN Auth List . . . . . . . . . . . . . . 4
3.1.1. OCSP Extension Specification . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. Privacy Considerations . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
The STIR problem statement [RFC7340] discusses many attacks on the
telephone network that are enabled by impersonation, including
various forms of robocalling, voicemail hacking, and swatting. One
of the most important components of a system to prevent impersonation
is the implementation of credentials which identify the parties who
control telephone numbers. The STIR certificates
[I-D.ietf-stir-certificates] specification describes a credential
system based on [X.509] version 3 certificates in accordance with
[RFC5280] for that purpose. Those credentials can then be used by
STIR authentication services [I-D.ietf-stir-rfc4474bis] to sign
PASSporT objects [I-D.ietf-stir-passport] carried in a SIP [RFC3261]
request.
The STIR certificates document specifies an extension to X.509 that
defines a Telephony Number (TN) Authorization List that may be
included by certificate authorities in certificates. This extension
provides additional information that relying parties can use when
validating transactions with the certificate. When a SIP request,
for example, arrives at a terminating administrative domain, the
calling number attested by the SIP request can be compared to the TN
Authorization List of the certificate that signed the request to
determine if the caller is authorized to use that calling number in
SIP.
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However, there is significant dynamism in telephone number
assignment, and due to practices like number portability, information
about number assignment can suddenly become stale. This problem is
especially pronounced when a TN Authorization List extension
associates a large block of telephone numbers with a certificate, as
relying parties need a way to learn if any one of those telephone
numbers has been ported to a different administrative entity.
No specific recommendation is made in the STIR certificates document
for a means of determining the freshness of certificates with a TN
Authorization List. This document explores approaches to real-time
status information for such certificates, and recommends an approach.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
3. Certificate Verification Methods
For traditional certificate status information, there are three
common certificate verification mechanisms employed by CAs:
1. Certificate Revocation Lists (CRLs) [RFC5280] (and [RFC6818])
2. Online Certificate Status Protocol (OCSP) [RFC6960], and
3. Server-based Certificate Validation Protocol (SCVP) [RFC5055].
When relying on status information, the verifier needs to obtain the
status information - but before that can happen, the verifier needs
to know where to locate it. Placing the location of the status
information in the certificate makes the certificate larger, but it
eases the client workload. The CRL Distribution Point certificate
extension includes the location of the CRL and the Authority
Information Access certificate extension includes the location of
OCSP and/or SCVP servers; both of these extensions are defined in
[RFC5280]. In all cases, the status information location is provided
in the form of an URI.
CRLs are an attractive solution because they are supported by every
CA. CRLs have a reputation of being quite large (10s of MBytes),
because CAs maintain and issue one monolithic CRL with all of their
revoked certificates, but CRLs do support a variety of mechanisms to
scope the size of the CRLs based on revocation reasons (e.g., key
compromise vs CA compromise), user certificates only, and CA
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certificates only as well as just operationally deciding to keep the
CRLs small. However, scoping the CRL introduces other issues (i.e.,
does the RP have all of the CRL partitions).
CAs in the STIR architecture will likely all create CRLs for audit
purposes, but probably not for real-time status information. Any
such CRLs used MUST be signed with the same algorithm as the
certificate. We thus anticipate that one of the two "online" options
is preferred. Between the two, OCSP is much more widely deployed and
this document therefore RECOMMENDS the use of OCSP in high-volume
environments (HVE) for validating the freshness of certificates,
based on [RFC6960], incorporating some (but not all) of the
optimizations of [RFC5019].
3.1. Using OCSP with TN Auth List
Certificates compliant with this specification SHOULD include a URL
[RFC3986] pointing to an OCSP service in the Authority Information
Access (AIA) certificate extension, via the "id-ad-ocsp" accessMethod
specified in [RFC5280]. It is RECOMMENDED that entities that issue
certificates with the Telephone Number Authorization List certificate
extension run an OCSP server for this purpose. Baseline OCSP however
supports only three possible response values: good, revoked, or
unknown. Without some extension, OCSP would not indicate whether the
certificate is authorized for a particular telephone number that the
verifier is validating.
At a high level, there are two ways that a client might pose this
authorization question:
For this certificate, is the following number currently in its
scope of validity?
What are all the telephone numbers associated with this
certificate, or this certificate subject?
Only the former lends itself to piggybacking on the OCSP status
mechanism; since the verifier is already asking an authority about
the certificate's status, that mechanism can be reused instead of
creating a new service that requires additional round trips? Like
most PKIX-developed protocols, OCSP is extensible; OCSP supports
request extensions (including sending multiple requests at once) and
per-request extensions. It seems unlikely that the verifier will be
requesting authorization checks on multiple telephone numbers in one
request, so a per-request extension is what is needed.
The requirement to consult OCSP in real time results in a network
round-trip delay, which is something to consider because it will add
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to the call setup time. OCSP server implementations commonly pre-
generate responses, and to speed up HTTPS connections, servers often
provide OCSP responses for each certificate in their hierarchy. If
possible, both of these OCSP concepts should be adopted for use with
STIR.
3.1.1. OCSP Extension Specification
The extension mechanism for OCSP follows X.509 v3 certificate
extensions, and thus requires an OID, a criticality flag, and ASN.1
syntax as defined by the OID. The criticality specified here is
optional: per [RFC6960] Section 4.4, support for all OCSP extensions
is optional. If the OCSP server does not understand the requested
extension, it will still provide the baseline validation of the
certificate itself. Moreover, in practical STIR deployments, the
issuer of the certificate will set the accessLocation for the OCSP
AIA extension to point to an OCSP service that supports this
extension, so the risk of interoperability failure due to lack of
support for this extension is minimal.
The OCSP TNQuery extension is included as one of the request's
singleRequestExtensions. It may also appear in the response's
singleExtensions. When an OCSP server includes a number in the
response's singleExtensions, this informs the client that the
certificate is still valid for the number that appears in the TNQuery
extension field. If the TNQuery is absent from a response to a query
containing a TNQuery in its singleRequestExtension, then the server
is not able to validate that the number is still in the scope of
authority of the certificate.
id-pkix-ocsp-stir-tn OBJECT IDENTIFIER ::= { id-pkix-ocsp 10 }
TNQuery ::= E164Number
The HVE OCSP profile [RFC5019] prohibits the use of per-request
extensions. As it is anticipated that STIR will use OCSP in a high-
volume environment, many of the optimizations recommended by HVE are
desirable for the STIR environment. This document therefore uses the
HVE optimizations augmented as follows:
o Implementations MUST use SHA-256 as the hashing algorithm for the
CertID.issuerNameHash and the CertID.issuerKeyHash values. That
is CertID.hashAlgorithm is id-sha256 [RFC4055] and the values are
truncated to 160-bits as specified Option 1 in Section 2 of
[RFC7093].
o Clients MUST include the OCSP TNQuery extension in requests'
singleRequestExtensions.
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o Servers MUST include the OCSP TNQuery extension in responses'
singleExtensions.
o Servers SHOULD return responses that would otherwise have been
"unknown" as "not good" (i.e., return only "good" and "not good"
responses).
o Clients MUST treat returned "unknown" responses as "not good".
o If the server uses ResponderID, it MUST generate the KeyHash using
SHA-256 and truncate the value to 160-bits as specified in Option
1 in Section 2 of [RFC7093].
o Implementations MUST support ECDSA using P-256 and SHA-256. Note
that [RFC6960] requires RSA with SHA-256 be supported.
o This removes the requirement to support SHA-1, RSA with SHA-1, or
DSA with SHA-1.
OCSP responses MUST be signed using the same algorithm as the
certificate being checked.
To facilitate matching the authority key identifier values found in
CA certificates with the KeyHash used in the OCSP response,
certificates compliant with this specification MUST generate
authority key identifiers and subject key identifiers using the
SHA-256 and truncate the value to 160-bits as specified in Option 1
in Section 2 of [RFC7093].
Ideally, once a certificate has been acquired by a verifier, some
sort of asynchronous mechanism could notify and update the verifier
if the scope of the certificate changes so that verifiers could
implement a cache. While not all possible categories of verifiers
could implement such behavior, some sort of event-driven notification
of certificate status is another potential subject of future work.
One potential direction is that a future SIP SUBSCRIBE/NOTIFY-based
accessMethod for AIA might be defined (which would also be applicable
to the method described in the following section) by some future
specification.
4. IANA Considerations
This document makes use of object identifiers for the TN-HVE OCSP
extension in Section 3.1.1 and the ASN.1 module identifier defined in
Appendix A. It therefore requests that the IANA make the following
assignments:
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TN-HVE OCSP extension in the SMI Security for PKIX Online Certificate
Status Protocol (OCSP) registry: http://www.iana.org/assignments/smi-
numbers/smi-numbers.xhtml#smi-numbers-1.3.6.1.5.5.7.48.1
5. Privacy Considerations
Querying for real-time status information about certificates can
allow parties monitoring communications to gather information about
relying parties and the originators of communications.
Unfortunately, the TNQuery extension adds a new field that could
potentailly be monitored by OCSP eavesdroppers: the calling telephone
number provides a specific piece of additional data about the
originator of communications. Using OCSP over TLS is one potential
countermeasure to this threat, as described in [RFC6960]
Appendix A.1.
Another way to mitigate leaking information about relying parties is
to use OCSP stapling. Strategies for stapling OCSP [RFC6961] have
become common in some web PKI environments as an optimization which
allows web servers to send up-to-date certificate status information
acquired from OCSP to clients as TLS is negotiated. A similar
mechanism could be implemented for SIP requests, in which the
authentication service adds status information for its certificate to
the SIP request, which would save the verifier the trouble of
performing the OCSP dip itself. Especially for high-volume
authentication and verification services, this could furthermore
result in significant performance improvements. This would however
require work on a generic SIP capability to carry OCSP staples that
is outside the scope of this document.
6. Security Considerations
This document is entirely about security. For further information on
certificate security and practices, see [RFC5280], in particular its
Security Considerations. For OCSP-related security considerations
see [RFC6960] and [RFC5019].
7. Acknowledgments
Stephen Farrell provided key input to the discussions leading to this
document. Russ Housley provided some direct assistance and text
surrounding the ASN.1 module.
8. References
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8.1. Normative References
[I-D.ietf-stir-certificates]
Peterson, J. and S. Turner, "Secure Telephone Identity
Credentials: Certificates", draft-ietf-stir-
certificates-11 (work in progress), October 2016.
[I-D.ietf-stir-passport]
Wendt, C. and J. Peterson, "Personal Assertion Token
(PASSporT)", draft-ietf-stir-passport-11 (work in
progress), February 2017.
[I-D.ietf-stir-rfc4474bis]
Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
"Authenticated Identity Management in the Session
Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-16
(work in progress), February 2017.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<http://www.rfc-editor.org/info/rfc3261>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC4055] Schaad, J., Kaliski, B., and R. Housley, "Additional
Algorithms and Identifiers for RSA Cryptography for use in
the Internet X.509 Public Key Infrastructure Certificate
and Certificate Revocation List (CRL) Profile", RFC 4055,
DOI 10.17487/RFC4055, June 2005,
<http://www.rfc-editor.org/info/rfc4055>.
[RFC5019] Deacon, A. and R. Hurst, "The Lightweight Online
Certificate Status Protocol (OCSP) Profile for High-Volume
Environments", RFC 5019, DOI 10.17487/RFC5019, September
2007, <http://www.rfc-editor.org/info/rfc5019>.
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[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC5912] Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
DOI 10.17487/RFC5912, June 2010,
<http://www.rfc-editor.org/info/rfc5912>.
[RFC6818] Yee, P., "Updates to the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 6818, DOI 10.17487/RFC6818, January
2013, <http://www.rfc-editor.org/info/rfc6818>.
[RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A.,
Galperin, S., and C. Adams, "X.509 Internet Public Key
Infrastructure Online Certificate Status Protocol - OCSP",
RFC 6960, DOI 10.17487/RFC6960, June 2013,
<http://www.rfc-editor.org/info/rfc6960>.
[RFC7093] Turner, S., Kent, S., and J. Manger, "Additional Methods
for Generating Key Identifiers Values", RFC 7093,
DOI 10.17487/RFC7093, December 2013,
<http://www.rfc-editor.org/info/rfc7093>.
[X.509] ITU-T Recommendation X.509 (10/2012) | ISO/IEC 9594-8,
"Information technology - Open Systems Interconnection -
The Directory: Public-key and attribute certificate
frameworks", 2012.
[X.680] ITU-T Recommendation X.680 (08/2015) | ISO/IEC 8824-1,
"Information Technology - Abstract Syntax Notation One:
Specification of basic notation".
[X.681] ITU-T Recommendation X.681 (08/2015) | ISO/IEC 8824-2,
"Information Technology - Abstract Syntax Notation One:
Information Object Specification".
[X.682] ITU-T Recommendation X.682 (08/2015) | ISO/IEC 8824-2,
"Information Technology - Abstract Syntax Notation One:
Constraint Specification".
[X.683] ITU-T Recommendation X.683 (08/2015) | ISO/IEC 8824-3,
"Information Technology - Abstract Syntax Notation One:
Parameterization of ASN.1 Specifications".
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8.2. Informative References
[RFC5055] Freeman, T., Housley, R., Malpani, A., Cooper, D., and W.
Polk, "Server-Based Certificate Validation Protocol
(SCVP)", RFC 5055, DOI 10.17487/RFC5055, December 2007,
<http://www.rfc-editor.org/info/rfc5055>.
[RFC6961] Pettersen, Y., "The Transport Layer Security (TLS)
Multiple Certificate Status Request Extension", RFC 6961,
DOI 10.17487/RFC6961, June 2013,
<http://www.rfc-editor.org/info/rfc6961>.
[RFC7340] Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
Telephone Identity Problem Statement and Requirements",
RFC 7340, DOI 10.17487/RFC7340, September 2014,
<http://www.rfc-editor.org/info/rfc7340>.
Appendix A. ASN.1 Module
This appendix provides the normative ASN.1 [X.680] definitions for
the structures described in this specification using ASN.1, as
defined in [X.680] through [X.683].
The modules defined in this document are compatible with the most
current ASN.1 specification published in 2015 (see [X.680], [X.681],
[X.682], [X.683]). None of the newly defined tokens in the 2008
ASN.1 (DATE, DATE-TIME, DURATION, NOT-A-NUMBER, OID-IRI, RELATIVE-
OID-IRI, TIME, TIME-OF-DAY)) are currently used in any of the ASN.1
specifications referred to here.
This ASN.1 module imports ASN.1 from [RFC5912].
[TO DO: this ASN.1 module is a stub and needs to be redone!]
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TN-Module-2016-2 {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-tn-module(88) }
DEFINITIONS EXPLICIT TAGS ::= BEGIN
IMPORTS
id-ad, id-ad-ocsp, id-pe -- From [RFC5912]
FROM PKIX1Explicit-2009 {
iso(1) identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-explicit-02(51) }
EXTENSION -- From [RFC5912]
FROM PKIX-CommonTypes-2009 {
iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkixCommon-02(57) }
;
id-pkix-ocsp OBECT IDENTIFIER ::= id-ad-ocsp
--
-- Telephone Number Query OCSP Extension
--
re-ocsp-tn-query EXTENSION ::= {
SYNTAX TNQuery IDENTIFIED BY id-pkix-ocsp-stir-tn }
TNQuery ::= E164Number
id-pkix-ocsp-stir-tn OBJECT IDENTIFIER ::= { id-pkix-ocsp 10 }
END
Authors' Addresses
Jon Peterson
Neustar, Inc.
Email: jon.peterson@neustar.biz
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Sean Turner
sn3rd
Email: sean@sn3rd.com
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